The survey, conducted by former Tory donor Lord Ashcroft, also spells trouble for David Cameron, with 11 of the remaining 12 constituencies on course to return a Labour MP.
Lord Ashcroft said the results showed Ukip was taking votes away from Labour voters as well as Tories as a result of strong work by activists on the ground.
He said: "With Nigel Farage planning to concentrate his party's resources in just 25 targets, Ukip's impact is likely to be felt in a series of local contests to which their headline vote share may bear little relation."
